The lining, protecting, or repair of the interior of tubes or tube-like structures has been a desirable expedient ever since tubes began to be subject to high stress applications. Some examples have been the use of tubes in hot water heaters, steam engines or steam generators. However, there are many other applications for the cladding and/or repair of tube-like structures. These would include fire arms, hydraulic pistons and the like. Further, cladding and surface repair can be used for a wide variety of exterior and irregular surfaces, such as those that would interact with hydraulic pistons and the like. Repair of special surfaces, such as chrome, is another potential use for cladding. Another application would be for plasticating tubes or barrels which often must endure substantial stresses or exposure to degrading substances.
Because of the temperatures and duty cycles involved in many of these applications, many metallic tube materials are rapidly degraded so that tube deterioration or non-functionality requires replacement of the tubes in question. When tube-like structures were sufficiently large, special metallic liners could be inserted to increase the operating life of the tube and to enhance functionality. However, as tubes grew ever smaller for a certain functions, the insertion of linings became more difficult.
Cladding the interior of a tube provided a much stronger and more durable interior surface than many external liner inserts could provide, and so became very popular for relatively large tubes that would be subject to high stress applications, such as plasticating barrels. A major problem with arc cladding or oxy acetylene torch cladding is that the surface developed thereby is extremely rough and irregular, requiring substantial machining (clean-up) to render the surface thickness sufficiently uniform and smooth for many uses.
Also, as tube sizes decreased, traditional arc or torch cladding became impossible. Conventionally, this meant that tube liners could be formed only near the two ends of the tube. Consequently, for many applications, cladding the interior of tubes was not an option.
This situation has been addressed to a large extent by the use of centrifugal casting to clad the interior of tubes. However, certain problems remained, especially with respect to plasticating barrels having relatively small diameters.
Extruders and tubers (rubber extruders) for high stress plasticating and plastic extrusion have been in use at least since the beginning of the twentieth century. With the vastly increasing use of plastics in the last half century, the demand for such extruders has become substantially greater, and the processing conditions have become far more severe. Originally plasticating devices were essentially a simple screw rotating inside a single-material barrel without a lining. This is no longer the case due to the more complex and corrosive materials being processed and the need for plasticating barrels having smaller diameters.
Both the barrel and screw of plasticating barrels are subject to wear from metal-to-metal contact and from abrasive and/or corrosive fillers in the plastic or rubber compounds. The earlier barrels had an internal surface that was nitrided to give improved abrasive wear resistance. In the later 1950's, bimetallic barrels were developed using a centrifugal casting process, as briefly described in the attached Spirex publication, entitled Plasticating Components Technology, 1997, incorporated herein by reference. Also, such improved barrels were adapted for use with injection molding machines in addition to conventional plastic extruder systems.
Centrifugal casting of plasticating barrels is a process used to line the internal surface of a barrel with an abrasion and/or corrosion resistant liner that is different from the barrel backing material or substrate. The process involves adding a lining material, such as a powder, inside the heavy-walled barrel cylinder at room temperature. The ends of the barrel are capped (usually cladded), and the barrel and unmelted powder are placed in a casting oven. The barrel is then rotated and heated until the liner materials are melted and uniformly distributed on the internal surface of the barrel. Early liner materials were iron/boron materials that created some metal carbides and were very much more wear resistant than conventional nitrited barrels. In 1968, improved liners became more abrasion resistant by the addition of very small, discrete metal carbide particles like tungsten carbide and/or equivalent materials.
Most rotational casting ovens are gas heated, but some are induction heated. In either case, the inside of the barrel must be heated to a point where the liner powder melts, but the thick-walled barrel material or substrate does not melt. After melting the powder is accomplished, the barrel is slowly cooled by radiation of the furnace heat into the surrounding environment, so that stresses are not induced and so that the liner material does not crack. After cooling, the barrel is honed, straightened, and machined to its final configuration and dimensions. There are a number of disadvantages to these techniques.
For example, often this process requires installation of a high-pressure sleeve at the discharge end of the barrel. Furnaces configured for rotating equipment are very expensive and require extensive maintenance. This includes periodic and prolonged shutdowns to reline the refractory surfaces of the oven and refurbish the rotating machinery. Further, even when the furnaces are functioning properly, set up for the coating of each barrel is an awkward and time-consuming process.
Also, the process of centrifugal coating requires that the liner material, or material matrix, melt at a lower temperature than the backing or substrate material. This creates severe limitations on the liner materials that can be used. As a result, abrasion-resistant and corrosion-resistant materials are limited to formulas that melt at a lower temperature than the barrel substrate. In many cases the optimum barrel substrate and liner materials preferred for the materials to be handled, cannot be used together. There is also the requirement of raising the backing or substrate material to a temperature close to its melting point, followed by a slow cooling to anneal the backing material to the liner material. This lowers the strength of the annealed backing material. Unfortunately, very high strengths are now required for many high stress applications, such as plasticating barrels. Such barrels can be subject to internal pressures of 40,000 psi or higher, and temperatures up to 700° F. These conditions often require the installation of a high pressure sleeve at considerable expense. Some newer, higher priced alloys can reduce this effect somewhat by reducing the loss of strength. However, greater expense is incurred, including greater processing times and more complex manufacturing facilities.
During the rotational casting process to form a barrel lining, the heavier metal carbide particles tend to be thrown outward by centrifugal force. This moves these particles away from the inside surface where they are needed for abrasion resistance. As a result, the resulting lining is far more susceptible to wear caused by abrasion than if the metal carbide particles are properly located on or very near the inner surface of the lining, or evenly distributed throughout the thickness of the lining or cladding.
Further, the high barrel temperatures that are reached during centrifugal casting make it difficult to maintain the barrel straightness and integrity, which are critical to the subsequent plastic processing operation. Straightening of the barrel cannot be done by conventional straightening presses because reverse bending cracks the relatively brittle liner. The rotational casting process requires a long time to heat up the liner and barrel substrate. Additional time is required for slow cooling (by radiating heat into the environment) after the lining operation. This causes added expense in labor and electrical costs.
Because a centrifugal lining process can only be successful in a very narrow range of processing times and temperatures, the results are often not satisfactory. High temperatures and long time periods spent at these temperatures cause dilution by migration of the substrate material into the barrel lining material or matrix. This causes reduced hardness and poor abrasion resistance. Also substrate migration of the base iron material can cause poor corrosion resistance in certain applications. Extended periods at high temperatures also cause the metal carbide particles coating the inner surface of the liner to melt into solution in the matrix matter (constituting the liner), rendering the carbide particles useless for anti-abrasion purposes.
When temperatures are too low and the time periods at proper processing temperatures are too short, an inadequate bond can result. Such an inadequate bond means that the liner may become separated from the barrel substrate or backing material. This condition could render the entire barrel useless for any purpose. Further, in some cases, portions of the liner may come dislodged, corrupting the molten plastic and/or fouling the screw pushing the molten plastic through the barrel. In either case, the barrel is subject to major failure, and the plastic processed therein ruined.
Even if the lined tube or barrel is not going to be used in plasticating applications, the dislodging of the liner with respect to the base material can prove problematic. For example, in boiler applications the base material of the tube could be prematurely corroded. Liquid running through the tube may be hindered to the point that essential heat transfer properties are compromised, and loose liner material could be stripped away, corrupting the overall water carrying system. Clearly, for these type applications, a strong metallurgical bond is needed and cannot be compromised.
Conventional MIG or TIG cladding of the inside diameter (ID) of barrels can be done to form metallurgical bonds, but it is more difficult to get such cladding heads into smaller diameter barrels. The area affected by heat is much greater than for laser cladding, and the cladded surface is poorer than that resulting from laser cladding. This results in far greater expense for post-clad finishing compared to the “near-net shape” of laser-cladding.
A totally different method to produce barrel liners is constituted by laser cladding. More specifically, laser cladding can bond liner material onto a base or substrate metal, forming a stable metallurgical bond. This new process diminishes or eliminates all of the disadvantages listed above, but has certain disadvantages of its own.
Laser cladding of the interior diameter (ID) of barrels, or other tube-like structures, involves the depositing of a liner material in the form of paste, or a separate liner tube, prior to cladding or during the cladding process. In the alternative, a powder or continuous wire can be applied. The laser cladding system is usually constituted by a laser beam delivered from a remote source via fiber optics and optical systems, or by a direct laser beam.
This technique has a number of clear advantages. For example, devices have been made that will allow laser cladding of tube diameters as small as 1 inch. Laser cladding also has a very shallow heat-affected depth which gives much less dilution of the liner material into the barrel substrate. This technique also creates much less stress in the substrate, reducing the tendency to bend or warp. Laser cladding also allows for a much more rapid cooling process since much less of the substrate of the barrel or tube has been heated to cladding temperatures. Rapid cooling has substantial advantages in overall manufacturing efficiency. However, rapid cooling can lead to difficulties under certain circumstances, as explained infra.
Laser cladding is a relatively robust process that allows a wide variety of material to be used, including materials that melt at higher temperatures than the barrel substrate. This can allow the use of improved matrix materials and improved ceramic or carbide materials as anti-abrasive coatings on the liner. Discrete abrasion resistant carbide or ceramic particles do not migrate toward the substrate as in a rotational casting. This leaves them more evenly distributed near the surface of the liner, where they are needed.
Further, the substrate does not necessarily need to be preheated prior to laser cladding, thus reducing production time and expense. Heat imparted by the laser cladding process is much more localized and can be quickly removed during the cladding process by a variety of internal and external methods. This means that a long cooling time can be eliminated. As a result, this cladding process is far less time-consuming than centrifugal casting.
Laser cladding is a process with a metallurgical bond, rather than a brazing process where the liner melts at a lower temperature than the substrate (to form a mechanical bond) as in rotational furnace casting. The laser cladding equipment is generally lower in cost than gas-fired or induction furnaces needed to contain the entire barrel and rotational equipment. Laser cladding is somewhat different than laser welding in that there is usually less than 2% dilution (migration between the liner material and the base material), but still sufficient to form a metallurgical bond. In contrast, welding is usually more than 5% or even 10% dilution of the weld matrix by migration from the base material. In certain applications, such as plasticating barrels, a true weld would be inappropriate because of the higher levels of migration or dilution. Only true cladding (generally less than a 2% dilution rate) is appropriate.
Several systems for laser cladding of the inside of pipes have been invented and commercialized. These include EPRI Patent Nos. 5,653,897 and 5,656,185 and IHI Patent No. 5,426,278. Also included are U.S. Pat. Nos. 5,496,422; 5,196,272; and, 5,387,292. All of the aforementioned patents are incorporated herein by reference to facilitate a better understanding of the present invention. These devices are designed to repair damaged or corroded heat exchanger tubes in power generation plants. More specifically, these systems are designed to make short, localized repairs in relatively long, fixed pipes that cannot rotate.
To accomplish cladding, each of these systems uses a rotating laser head. The systems described in the aforementioned patents include the insertion of a cladding or inlay material by wire, powder, paste, or thin wall tube. The paste and the tubes are already in place before laser cladding. In the case of the EPRI patents, a coiled wire is placed inside the pipe directly above the repair area in order to have it easily accessible and easy to feed as the cladding proceeds. This method is limited to short longitudinal lengths of clads, as are generally required in boiler repair. Powder is difficult to introduce in the horizontal position because, without gravity assist, it tends to clog and interrupts cladding. Drawings of these various cladding systems are shown in the respective patents.
For prolonged or full length cladding of 20:1 (length-to-diameter) or longer pipes, tubes, and barrels, the cladding head, and especially the reflecting mirrors associated with it, must be cooled. This can be done by a cooling fluid such as air or water. The EPRI patent does not have such cooling except for the bearings that are required to rotate the head inside the pipe. The IHI device allows cooling (by air) coming from the direction of the laser source.
All of the aforementioned systems must have extensive auxiliary services introduced from the laser head end of the tube because access from the opposite end is not available and cannot be coordinated with the activity provided from the laser end. These auxiliary services can include fiber optical viewer, wire/powder feeds, cooling media, optics (lenses) and focusing devices.
There is also a need to make such devices ever smaller. In particular, barrel IDs as small as 14 mm (0.551 inch) are used for plasticating barrels. Thus, appropriate cladding devices are necessary to clad or line the interior of the plasticating barrels. Conventional rotating cladding devices operate entirely from one end of the tube being lined or cladded. Consequently, size reduction for such cladding devices is severely limited. This is particularly true since the cladding head must include all auxiliary services, as well as the bearings required for rapid rotation of the cladding head. This entire structure is fed into the tube to be cladded from only one side of the tube. As a result, size reduction of the overall cladding apparatus is very problematical, and cannot accommodate some smaller sizes used for plasticating barrels.
The aforementioned difficulties in conventional systems, the inability to produce smooth, uniform linings for plasticating barrels has been addressed in large part by the systems disclosed in U.S. Pat. No. 6,486,432 to Colby et al., issued Nov. 26, 2002, and incorporated herein by reference in its entirety. This patent provides a wide variety of different approaches to obtaining smooth, uniform linings in tubular structures such as plasticating barrels.
However, there are number of drawbacks that still exist. For example, in a horizontally-positioned tube, smoke and debris tend to accumulate due to the force of gravity, especially if the tube is long and the cladding head, rather than the tube, is moving. This accumulation of smoke and debris compromises the speed of the cladding process, results in wasted cladding powder, and even degrades the cladding due to accumulated debris, which can occur even in a spinning tube. Also, in horizontally aligned tubes, the best cladding angles can sometimes be hard to obtain, thereby resulting in lost energy and a slower cladding process. Reflection of laser energy from the smooth, cladded surface that is the object of laser cladding, can also damage equipment as well as waste energy.
Carbide particles are often added to the clad to present a very hard, anti-abrasive surface that is necessary with many of the materials being processed in plasticating screws. Unfortunately, conventional cladding techniques permit migration of these carbide particles from the surface of the clad to positions deeper in the clad, where the particles are far less effective. Also, conventional cladding techniques usually result in the loss of many of the carbide particles used in the process, thereby entailing substantial unnecessary expense. Particles within conventional clads also necessitate that substantial post-clad finishing or polishing techniques be used. Such techniques can be very extensive in nature and can greatly raise the price and complexity of the overall manufacturing process. It is important to note that the extent of post-clad machining or polishing (to smooth and obtain uniform liner thickness) is a major operational factor in the lining of tubes and barrels, especially plasticating barrels.
Also, there is the well-known problem of cracked laser clads due to overly-rapid cooling or uneven cooling. This is not addressed in the examples of conventional art, and can be a major problem with laser cladding, depending upon the types of materials used in the cladding process. Further, the well-known difficulties of addressing multi-bore plasticating barrels still exist with the conventional systems.
Accordingly, an improved laser cladding system for barrels, tube-like structures, and other surfaces would address all of the aforementioned drawbacks of conventional systems while providing smooth, uniform linings for even small diameter barrels. The improved laser cladding system would also provide a capability for efficiently cladding double-bore barrel configurations, and providing a repair system for a wide variety of surfaces and materials, such as chrome plating.